(a) Field of the Invention
The present invention relates to a high output power semiconductor laser diode and, more particularly, to a semiconductor laser diode formed on a GaAs substrate and having an active layer including at least indium (In), gallium (Ga) and arsenic (As).
(b) Description of the Related Art
Today, semiconductor laser diodes are increasingly used in a variety of fields, and the request for higher output power semiconductor laser diodes is also increasing.
For example, in an optical communication field, the output power of a pumping light source used for pumping an erbium-doped fiber amplifier (EDFA) was as low as several tens of milli-watts in the beginning of the 1990""s when it was developed for practical use. However, along with the recent remarkable development of the optical communication technology, such as wavelength division multiplexing (WDM), the output power requested for the current semiconductor laser diode is far higher than 100 milli-watts, in addition to the operational lifetime being requested at one million hours for the semiconductor laser diode.
FIG. 1 shows a conventional semiconductor laser diode (sometimes referred to as simply xe2x80x9claser diodexe2x80x9d hereinafter) which is generally used as a pumping light source for an EDFA. The laser diode, generally designated by numeral 60, includes an n-type GaAs (n-GaAs) substrate 12 and a resonant cavity formed thereon as a layer structure. The layer structure includes an n-AlGaAs cladding layer 14, an InGaAs/GaAs multiple-quantum-well (MQW) active layer structure 62, a p-AlGaAs cladding layer 22, and a p-GaAs cap layer 24 consecutively formed on the n-GaAs substrate 12. The resonant cavity has an emission wavelength of 980 nm.
To adjust the emission wavelength at 980 nm, each InGaAs quantum well layer of the MQW active layer structure 16 has an indium (In) atomic ratio of 0.16 to 0.20 with respect to the total amount of III-group atoms, and has a compressive strain in the range between 1.1% and 1.4%. Here, and throughout this document, we use the conventional definition of the strain (xcex5) of the well layer with respect to the substrate. The strain may be computed from the average lattice constants of the well material and the substrate material as follows:
strainxcex5 (in percentage)=100%xc2x7 (Awxe2x80x94Asub)/Asub,
where Aw is the average lattice constant of the material of the well layer, and where Asub is the average lattice constant of the material of the substrate. A negative value indicates tensile strain, while a positive value indicates compressive strain. Because the layers that are disposed between the substrate and the well layer are relatively thin, these intermediate layers conform to the average lattice constant of the substrate, and effectively present the average lattice constant of substrate to the well layer.
As another point of terminology, the atomic ratio is also known as the xe2x80x9cmole fraction.xe2x80x9d The atomic ratio of an element in a III-V composition is the amount of that element divided by the total amount of the elements in the composition which are from the same group (e.g., column) of the periodic table. In addition, the amount of an element in a III-V material composition may also be specified as a content percentage.
The content percentage is 100 times the corresponding atomic ratio. For example, an atomic ratio of 0.16 for indium is equivalent to an indium content of 16%.
Among the layer structure, the cap layer 24 and the top portion of the cladding layer 22 are configured to form a mesa structure having a ridge-stripe waveguide. On the side surfaces of the mesa structure and the top of the cladding layer 22, a SiN film 28 is formed except for an opening 26 which exposes therefrom the top surface of the cap layer 24. A p-side electrode 30 made of metallic films including Ti/Pt/Au films is formed on the SiN film 28 and in contact with the cap layer 24 through the opening 26. An n-side electrode 32 made of metallic films including AuGeNi/Au films is formed on the bottom surface of the GaAs substrate 12.
The strong demand for the WDM system requests year by year a higher output power of the pumping light source used for the EDFA. The conventional semiconductor laser diode has an operational lifetime as long as one million hours in the output power region lower than 100 milli-watts. However, the conventional laser diode suffers from a far shorter lifetime in the output power region higher than 100 milli-watts, thereby exhibiting a lower reliability with respect to the long-term operation.
In view of the above, it is an object of the present invention to provide a 980-nm-band semiconductor laser diode which is capable of operating with a higher reliability in a longer lifetime at a higher output power region.
The present invention provides, in a first broad aspect thereof, a semiconductor laser device including: a GaAs substrate having a top surface; and a resonant cavity having a layer structure formed on the GaAs substrate and oriented to guide light in a direction parallel to the top surface of the GaAs substrate, the layer structure having an active layer which generates light when an electric current is passed through it. The active layer has a light-generation layer which comprises a first amount of group-III atoms and a second amount of group-V atoms, the first amount of group-III atoms including at least gallium (Ga) and the second amount of group-V atoms including at least arsenic (As). The light-generation layer further includes at least one of the elements of indium and nitrogen, the indium being limited to an atomic ratio in the range of 0.0 to 0.10 with respect to the amount of group-III atoms, and the nitrogen being limited to an atomic ratio in the range of 0.0 to 0.025 with respect to the amount of group-V atoms. As used herein, a light-generation layer is a layer in which electromagnetic radiation is generated. A light-generation layer may comprise a well layer disposed between two barrier layers (e.g., a quantum well structure), or may comprise a layer disposed between two optical guiding layers. In each of these examples, the conduction-band energy level of the light-generation layer is typically lower than the conduction-band energy level of the barrier layers or the optical guiding layers.
In one embodiment of the present invention constructed for lasing operation near 980 nm (that is, having a photoluminescence peak near 980 nm for the light-generation layer), the layer structure further comprises an AlzGa1-zAs barrier layer, and the light-generation layer comprises a GaxIn1-xAsyN1-y well layer disposed adjacent to the AlzGa1-zAs barrier layer, with x, y and z satisfying the relationships: 0xe2x89xa61-xxe2x89xa60.05, 0 less than 1-yxe2x89xa60.011 and 0.03 xe2x89xa6zxe2x89xa61, respectively. The layer structure further comprises one or more cladding layers to optically confine the light generated by the light-generation layer. To enable lasing operations between 980 nm and 1100 nm (to provide a photoluminescence peak for the light-generation layer in the range of 980 nm to 1100 nm), the upper limit of the nitrogen atomic ratio (1-y) may be raised to 0.025 (2.5% content), and the minimum aluminum atomic ratio in the barrier layer should be increased in proportion from 0.03 (3% content) at 980 nm laser emission to 0.13 (13% content) at 1100 nm laser emission. For lasing operations between 980 nm and approximately 940 nm, the minimum aluminum atomic ratio in the barrier layer can be decreased from 0.03 (3% content) at 980 nm laser emission to 0 (0% content) at approximately 940 nm laser emission.
In another embodiment of the present invention constructed for lasing operation near 980 nm (that is, having a photoluminescence peak near 980 nm for the light-generation layer), the layer structure further comprises a Gax2In1-x2Asy2P1-y2 barrier layer, and the light-generation layer comprises a Gax1In1-x1Asy1N1-y1 well layer which is disposed adjacent to the Gax2In1-x2As2y barrier layer, with x1, x2, y1 and y2 satisfying the relationships 0xe2x89xa61-x1xe2x89xa60.05, 0 less than x2 less than 1, 0 less than 1-y1 less than 0.011 and 0 less than y2 less than 1, respectively. The Gax2In1-x2Asy2P1-y2 barrier layer is preferably substantial lattice-matched to the GaAs substrate (magnitude of strain less than 0.2%), and preferably has a bandgap wavelength of 0.85 xcexcm or less. The layer structure further comprises one or more cladding layers to optically confine the light generated by the light-generation layer. To enable lasing operations between 980 nm and 1100 nm (to provide a photoluminescence peak for the light-generation layer in the range of 980 nm to 1100 nm), the upper limit of the nitrogen atomic ratio (1-y) may be raised to 0.025 (2.5% content), and upper limit of the bandgap wavelength of the barrier layer should be decreased in proportion from 0.85 xcexcm at 980 nm laser emission to 0.805 xcexcm at 1100 nm laser emission. For lasing operations between 980 nm and approximately 940 nm, the upper limit of the bandgap wavelength of the barrier layer can be increased from 0.85 xcexcm at 980 nm laser emission to 0.87 xcexcm at approximately 940 nm laser emission.
In yet another embodiment of the present invention constructed for lasing operation near 980 nm (that is, having a photoluminescence peak near 980 nm for the light-generation layer), the layer structure further comprises an AlzGa1-zAs barrier layer, and the light-generation layer comprises a GaxIn1-xAs1-y1-y2Ny1Sby2 well layer which is disposed adjacent to the AlzGa1-zAs barrier layer, with x, y1, y2 and z satisfying the relationships 0xe2x89xa61-xxe2x89xa60.05, 0xe2x89xa6y1xe2x89xa60.011,0 less than y2 less than 0.04 and 0.03xe2x89xa6zxe2x89xa61, respectively. The layer structure further comprises one or more cladding layers to optically confine the light generated by the light-generation layer. In this embodiment, it is noted that the known upper limit of the Sb content for the improvement of the crystallinity during growth of the GaInNAs layer is 1.6% if the nitrogen content therein is 0.44%, as described in an earlier application No. JP-2001-124300. Since the upper limit of the nitrogen atomic ratio y1 in the above formula GaxIn1-xAs1-y1-y2Ny1Sby2 of the well layer is 0.011 (0xe2x89xa61-xxe2x89xa60.05, 0 less than y1xe2x89xa60.011), corresponding to a content percentage of 1.1%, the upper limit of the Sb content (corresponding to atomic ratio Y2) necessary for improvement in this well layer is calculated as 4% as follows:             1.6      ⁢      %      xc3x97      1.1      ⁢      %              0.44      ⁢      %        =      4    ⁢    %  
Thus, the relationship 0 less than y2  less than 0.04 in the GaxIn1-xAs1-y1-y2Ny1Sby2 well layer is obtained. To enable lasing operations between 980 nm and 1100 nm (to provide a photoluminescence peak for the light-generation layer in the range of 980 nm to 1100 nm), the upper limit of the nitrogen atomic ratio (y1) may be raised to 0.025 (2.5% content), and the minimum aluminum atomic ratio in the barrier layer should be increased in proportion from 0.03 (3% content) at 980 nm laser emission to 0.13 (13% content) at 1100 nm laser emission. The upper limit of Sb atomic ratio y2 would accordingly increase from 0.04 (4% content) at a nitrogen atomic ratio of 0.011 (1.1% content) to 0.091 (9.1% content) for a nitrogen atomic ratio of 0.025 (2.5% content). For lasing operations between 980 nm and approximately 940 nm, the minimum aluminum atomic ratio in the barrier layer can be decreased from 0.03 (3% content) at 980 nm laser emission to 0 (0%) at approximately 940 nm laser emission.
In still another embodiment of the present invention constructed for lasing operation near 980 nm (that is, having a photoluminescence peak near 980 nm for the light-generation layer), the layer structure further comprises a Gax2In1-x2ASy2P1-y2 barrier layer, and the light-generation layer comprises a Gax1In1-x1As1-y1-y2Ny1Sby2 well layer which is disposed adjacent to the Gax2In1-x2Asy3P1-y3 barrier layer, with x1, x 2, y1, y2 and y3 satisfying the relationships 0xe2x89xa61-x1xe2x89xa60.05, 0 less than x2  less than 1, 0 less than y1xe2x89xa60.011, 0 less than y2 less than 0.04 and 0 less than y3 less than 1, respectively. The Gax2In1-x2ASy3P1-y3 barrier layer is preferably substantially lattice-matched to the GaAs substrate (magnitude of strain less than 0.2%), and preferably has a bandgap wavelength of 0.85 xcexcm or less. The layer structure further comprises one or more cladding layers to optically confine the light generated by the light-generation layer. To enable lasing operations between 980 nm and 1100 nm (or a photoluminescence peak for the light-generation layer in this range of wavelengths), the upper limit of the nitrogen atomic ratio (1-y) may be raised to 0.025 (2.5% content), and upper limit of the bandgap wavelength of the barrier layer should be decreased in proportion from 0.85 xcexcm at 980 nm laser emission to 0.805 xcexcm at 1100 nm laser emission. The upper limit of Sb atomic ratio y2 would accordingly increase from 0.04 (4% content) at a nitrogen atomic ratio of 0.01 1 (1.1% content) to 0.091 (9.1% content) for a nitrogen atomic ratio of 0.025 (2.5% content). For lasing operations between 980 nm and approximately 940 nm, upper limit of the bandgap wavelength of the barrier layer can be increased from 0.85 xcexcm at 980 nm laser emission to 0.87 xcexcm at approximately 940 nm laser emission.
In preferred implementations of each of the above embodiments, the atomic ratio of gallium (Ga) with respect to the amount of group-III atoms in the light-generation layer is equal to or greater than 0.5, and the atomic ratio of arsenic (As) with respect to the amount of group-V atoms in the light-generation layer is equal to or greater than 0.5. Also, the preferred implementations are conventional pumping laser light sources which output at least 100 mW of optical power through a surface portion of the laser""s front facet, with the surface portion having an area which is less than 100 square micron.
In accordance with the semiconductor laser device of the present invention, the laser diode operates with a higher reliability in a long-term operation at a higher output power, owing to the specific compositions of the light-generation layers which allow for a reduction in the strain of the active layer with respect to the GaAs substrate.